The present disclosure relates to all-reflective optical systems and in particular to an all-reflective afocal optical system, such as an all-reflective afocal telescope, that is derived from the first two mirrors of a focal three-mirror anastigmat optical system, such as a focal three-mirror anastigmat telescope.
An example of a conventional focal three-mirror anastigmat telescope is illustrated by U.S. Pat. No. 4,101,195 to Korsch, entitled “Anastigmatic Three-Mirror Telescope”, the entire contents of which is incorporated herein by reference. The patent discloses a focal three-mirror telescope having an ellipsoidal primary mirror, a hyperbolic secondary mirror, and an ellipsoidal tertiary mirror, with the three mirrors producing an image in a conveniently located finite plane for viewing.
An example of a conventional all-reflective afocal telescope is illustrated by U.S. Pat. No. 3,674,334 to Offner, entitled “Catoptric Anastigmatic Afocal Optical System”, the entire contents of which is incorporated herein by reference. The patent discloses a catoptric anastigmatic afocal optical system comprising two concave paraboloidal reflectors of different powers for the primary and tertiary mirrors and a convex hyperboloidal secondary mirror reflector adapted to receive and recollimate radiant energy.
Focal and afocal telescopes are individually best suited for different imaging tasks. For example, certain imaging tasks such as wide area search functions are preferably done in a vehicle scanning mode using linear focal plane array (FPA) technology. A focal three-mirror anastigmat telescope may be best suited for these tasks. Also, certain other imaging tasks such as point collects for detailed interrogation and characterization are best done behind an afocal optical system because such tasks use custom instruments with two-dimensional (2D) staring FPA's or internally scanned linear FPAs. An afocal telescope may be best suited for these tasks.
Thus, depending on the imaging tasks to be performed, conventionally different telescopes have to be used. What is needed is an all-reflective optical system that may be constructed to provide both focal and afocal optical functions.